1. Field of the Invention
The invention concerns a ferruginous catalyst for decreasing the content of nitrogen oxides in flue gases.
2. Background Information
Nitrogen oxides (NO.sub.x) are generated both from the nitrogenous constituents of the fuel and from the nitrogen in the air when fossil fuels are burned. The oxides enter the atmosphere and become extremely detrimental to the environment. Nitrogen oxides are considered partly responsible for the destruction of forests by "acid rain", and the formation of "photochemical smog" is in particular ascribed to nitrogen oxides.
There exists, due to the constant increase in NO.sub.x emissions in recent years, a high level of interest in decreasing the content of NO.sub.x in flue gases.
It is known that nitrogen oxides can be converted into N.sub.2 and H.sub.2 O by NH.sub.3 and that the reaction is fairly selective over a wide range of temperatures, meaning that, since it proceeds in the presence of a high excess of oxygen (as is usual in flue gases) without excessive loss of ammonia as the result of oxidation, only relatively small amounts of reductants are necessary. Various catalysts for reducing NO.sub.x with ammonia are also known.
The catalytically active constituents of these known NO.sub.x -reduction catalysts are mainly toxic heavy-metal compounds. Thus, German Pat. No. 2 458 888 specifies a combination of titanium dioxide with oxides of the metals vanadium, molybdenum, tungsten, nickel, cobalt, uranium, etc. Since the action of the nitrogen-decreasing catalysts constantly loses its effectiveness the longer the processing continues under the operating conditions typical of this field of application, they must be replaced after about two to five years. The no longer usable catalysts, which contain high levels of toxic constituents, must then be expensively regenerated or disposed of subject to strict controls. Given the huge amounts of catalyst required for, say, decreasing the nitrogen content of power-plant flue gases, the risk of secondary environmental damage is obvious.
The use of molecular sieves to catalyze NO.sub.x reduction with NH.sub.3 is also known. German OS No. 3 000 383, for example, employs a clinoptilolith in the form of hydrogen as a catalyst. The form is obtained by ion exchange of a naturally occurring clinoptilolith with a solution of ammonium nitrate and washing with a powerful acid.
German OS No. 3 328 653 specifies a catalyst consisting of a ceramic molecular sieve with permeability cross-sections that have diameters ranging from shorter than the critical molecular diameter of ammonia to longer than that of nitrogen.
The catalytic action of these molecular-sieve catalysts is accordingly determined by their pore structures. The catalysts entail the risk of damage to crystalline structure at high reaction temperatures due to the effect of the water vapor present in high concentration in the flue gases, whereas at low temperatures the ready absorption of the vapor leads to decreased activity. An especially serious drawback to molecular sieves like H-mordenite and H-clinoptilolite is the formation of considerable concentrations of undesirable N.sub.2 O, as known, for example, from the research of J. R. Kiovsky, P. B. Koradla, and C. T. Lin, Ind. Eng. Chem. Prod. Res. Dev., (1980), 218.
A similar reduction catalyst for removing nitrogen oxides from flue gases is known from German AS No. 2 446 006. It consists of an aluminum-silicate hydrate containing 1 to 10% alkali or alkaline-earth oxide, of, that is, a synthetic zeolite, as a carrier, and of a salt, oxide, or hydroxide of iron as a catalytic constituent. This catalyst is, due to its high content of alkali or alkaline earth, not very resistant to sulfur.
The use of iron oxide as a catalyst for the reduction of NO.sub.x is known from German Pat. No. 2 621 944, which employs iron ore with a particle size of 3 to 35 mm. The used catalyst is continuously removed and transferred to a blast furnace for use as a raw material in iron production. This catalyst is inappropriate for stationary-bed reactors because it leads to unacceptably high pressure loss.
German Pat. No. 2 460 681 also specifies an iron-oxide reduction catalyst, obtained by magnetic precipitation from blast-furnace dusts, sintering-plant dust, etc. Its relatively short life is especially undesirable considering that replacing a charge of catalyst is very time-consuming and expensive.
It is also known, from German Pat. No. 2 619 662, that Fe.sub.2 O.sub.3 obtained from alpha-goethite by thermal dehydration will catalyze NO.sub.x reduction. The only appropriate starting material, of course, is of course this oxide-hydrate modification, and the resulting iron oxide has a very weak action. Furthermore, according to German Pat. No. 2 639 848, hematite obtained from goethite will only exhibit sufficient catalytic action when the ratio of reflex intensities obtained by X-ray diffraction at specific planes in the crystal lattice exceeds a prescribed level.
German Pat. No. 2 525 880 specifies a porous catalyst for removing nitrogen oxides that consists of iron oxide or iron sulfate as active constituents and of a SiO.sub.2 -Al.sub.2 O.sub.3 carrier with a SiO.sub.2 content of at least 60% and a porosity that includes at least 0.15 ml/g with pores greater than 15 nm. The obvious drawback to this catalyst is that the carrier must be produced subject to precisely maintained conditions from, say, a solution of Al(SO.sub.4).sub.3 and a hydrogel of SiO.sub.2 by neutralization with NH.sub.3, which is an expensive method.
German OS No. 2 784 471 specifies a catalyst paste for use in the vapor-phase reduction of nitrogen oxides with ammonia. The paste contains iron oxide on a shaped carrier made out of titanium dioxide and a clay mineral with a mean particle size of 0.1 to 100 .mu.m. The clay mineral acts as a binder for the titanium-dioxide particles and is intended to accordingly improve the catalyst's crush strength. The clay mineral has no catalytic action. The catalytic action is primarily ascribed to an interaction between the iron oxide and the titanium dioxide.
German OS No. 2 504 027 specifies a method for selectively reducing nitrogen oxides in flue gases that, in addition to nitrogen oxides, also contain sulfur oxides in the presence of a catalyst that contains iron sulfate on a carrier, with ammonia employed as a reducing agent. The carriers are preferably inert refractory carriers like aluminum oxide, cordierite, and mullite, that do not enter into a catalytic interaction with the iron sulfate. The presence of iron sulfate is considered essential because iron nitrate, for example, results in useless catalysts.
The object of the prior pending application U.S. Ser. No. 880,168 is a catalyst for decreasing the nitrogen-oxide content of flue gases that contains an active constituent in the form of an acid aluminosilicate with a layered structure. A catalyst of this type, which can be obtained by, among other methods, treating appropriate starting materials that contain layer silicates with aqueous mineral acid, is very active in reducing nitrogen oxides to H.sub.2 O and N.sub.2 with ammonia.